There are actually a few scripts that you can use that would let them safely and easily edit online ONLY THE PARTS OF THE PAGE you want them to.

We've posted about a couple of them in the past but i don't have any particular experience with them and can't remember the names -- hopefully a follow up poster will.

I do know they are designed for basically the exact case you are talking about -- where you have a non-technical client who need to be able to do regular minor editing of some page content, without letting them mess up the entire web design.